Freezing tunnels for use in recycling scrap are known. An exemplary tunnel was proposed by the present inventor in Canadian patent No. 1,136,594 which issued on Nov. 30, 1982. That patent describes an insulated freezing tunnel which is inclined and rotated to convey scrap from a receiving end of the tunnel to a discharge end of the tunnel. Liquid nitrogen is injected into the tunnel and freezes the scrap during transit. At the discharge end of the tunnel, the scrap might typically be delivered to a hammer mill which shatters the frozen materials.
The cost of operating a freezing tunnel is determined in large measure by the quantity of liquid cryogen required. In the past, nitrogen in gaseous form has been allowed to escape freely from the tunnel and such discharge would occur before the gas was fully spent. It would be desirable to avoid such waste of cryogenic material and to provide more thorough transfer of heat to the spent gas before discharge.
Another shortcoming associated with prior freezing tunnels is actual loss of liquid cryogen at the discharge end of the tunnel. The scrap locates principally at the bottom of the tunnel and is often immersed in liquid cryogen. At the discharge end of the tunnel, the liquid cryogen escapes with the scrap into the associated hammer mill. Reducing loss of liquid cryogen in this manner is difficult as obstruction of liquid flows necessarily obstructs the discharge of solid scrap.
Another problem with prior practices is clinging of fine scrap, such as certain plastics, to the walls of the tunnel. The scrap can be lifted from the tunnel walls by providing ribs to enhance tumbling action and by increasing the rate of tunnel rotation. However, increasing the rate of tunnel rotation incidentally increases the rate of transport of the materials and the residence time within the tunnel. This hampers the ability to process scrap at appropriate speeds.
The present invention in its various aspects addresses these and other shortcomings associated with prior freezing tunnels.